Funny how often science discovers things it didn't know it was looking for.

Twenty years ago, Canadian researcher David Bailey was preparing to study alcohol's effect on the blood pressure medicine felodipine. To properly “blind” participants, Bailey and his wife rummaged through their fridge looking for something that would mask the taste of the alcohol. They settled on grapefruit juice.

Once the study began, however, Bailey was surprised to discover concentrations of felodipine in the volunteers' blood were many times higher than expected. Suspecting the grapefruit juice was somehow to blame (and in a decision either brave or foolhardy), Bailey tested his hypothesis by running an experiment with a single subject: himself.

His study, published in the medical journal The Lancet in 1991, has since been replicated many times and today, grapefruit's ability to cause prescription drugs to build up in the body is a medical fact. It happens because substances in grapefruit juice disrupt enzymes in the body that metabolize, or break down, drugs. Without the enzyme, the drug keeps accumulating, sometimes to dangerous levels.

Bailey, today a senior scientist at Lawson Health Research Institute in London, Ontario, recently published a follow-up article in the Canadian Medical Association Journal warning that as new drugs are introduced, the number that may interact with grapefruit juice is skyrocketing. In 2008 there were 17 drugs that Bailey calls “worrisome.” Today there are 44, an average of about six new drugs a year.

“Taking these prescription drugs with grapefruit juice is like taking several tablets at once,” says Bailey, adding that this is perhaps the most profound food-drug interaction known. (Other citrus fruits that can have the same effect include Seville oranges, pomelos and limes.)

Depending on the drug, side effects can include gastrointestinal bleeding, a heart rhythm disruption that goes by the pleasant-sounding name torsade de pointes, abnormally low blood pressure and sudden cardiac death. And it doesn't take much. Drinking as little as 6-7 ounces of grapefruit juice can hamper drug metabolism for as long as 72 hours.

At greatest risk? The elderly, who tend to take more prescription drugs and, yes, consume more grapefruit.

While computer programs help alert physicians and pharmacists about drug-on-drug interactions, preventing food-drug interactions is trickier.

“Adding a question about grapefruit juice to a patient's pharmaceutical profile is certainly something worthy of consideration,” says Oralia Bazaldua, a pharmacist with the University of Texas Health Science Center. “We shouldn't scare people, but we should make them more aware of the problem.”

Bailey doesn't prescribe an across-the-board prohibition of grapefruit. Instead, he's developed a list of alternative drugs that don't interact with grapefruit juice. Patients can discuss these with their doctor. You can find the list here: http://bit.ly/So0w8c.

It may sound scary, but it's better to know you're at risk. While researching this column I discovered the simvastatin I take for high cholesterol can cause kidney failure when combined with grapefruit juice. My doctor never mentioned a thing.